[Doc'd] Update Documentation with the recommendation that THP should be disabled on Couchbase nodes.

Nodes on which THP(Transparent Huge Pages) are enabled, we are seeing high swap usage and severe timeouts in ns_server. This may get much worse under memory pressure. So ns_server had to restart whole bunch of services because of random timeouts and in the end it reached max restart intensity and therefore terminated.

So by default THP must be disabled and this information needs to be documented.